Question about Experiment of recrystallization of a solid 1. Explain the purpose

Question

Question about Experiment of recrystallization of a solid

1. Explain the purpose of the following in this experiment:

a. using hot filtration

b. not cooling (chilling) the hot filtered solution immediately

c. washing the crystals in the final step with ice cold water

d. using gravity filtration (not vacuum filtration) in the hot filtration step

e. using a stemless funnel in the hot filtration step

2. What are the requirements for a good recrystallizing solvent?

3. Using the data of Table 6.1, compare the following pair of liquids as recrystallization solvents:

a. water vs. acetone

b. methanol vs. toluene

c. benzene vs. cyclohexane

d. chloroform vs. ethyl acetate

Explanation / Answer

1 a. Any impurities which is undissolved in hot solvent is seperated by filteration, while the solution is still hot

b. The crystallization is slow process, on cooling the mixture very rapidly will yield crystals of long range orders.

c. As the recrystallization is based on the fact the compond is soluble in solvent while its hot, where as partially or insoluble in same solvent when its cold. So in order to avoid dissolution of crstals formed, it is wahed with cold water.

d. Vaccum filteration cannot be used because reduced pressure in suction flask will cause the filterate to boil and material in solution will get deposited over the walls of the flask.

e. To avoid problem of crystallization and subsequent clogging in the stem of the funnel, the use of stemless funnel is recommended.

2. Requirement of good crstallization solvent

a. Find a solvent which will dissolve the crude materials readily while its hot, but only to a small extent when its cols.

b. Melting point of compond must be lower than the boiling point of solvent

c. If pure solvent is not suitable mixture of solvent can be employed for same.

3. a water is preferred over acetone, as acetone is more flammable and toxic.

b. Methanol is preferred, as it is less toxic than toluene.

c. Cyclohexane is preferred, as benzene is highly toxic in nayure

d. Ethylacetate is preferred as chloroform id highly toxc in nature

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